In a pressurized water reactor ("PWR") a pressure vessel within which the reactor core is located sits in a reactor cavity surrounded by a structural wall (typically concrete) known as the primary shield. The pressure vessel has a number of radially extending nozzles through which coolant enters and leaves the vessel, and is configured with a removable head to allow access to the core. The upward extensions of the primary shield above the head define a refueling canal which is flooded so that refueling operations may be carried out under water. The refueling canal walls extend upwardly to an operating deck which provides access to other components of the reactor complex. A typical height of the operating deck is about 20 feet above the level of the joint between the pressure vessel and the head.
For safety reasons, it is typically a requirement that the pressure vessel and nozzles be visually inspected during shutdown. To allow such inspection, the primary shield is spaced radially outwardly from the pressure vessel to define an annular gap, which may be on the order of 2-3 feet in radial extent. During refueling, the pressure vessel is sealed to the primary shield at the level of the pressure vessel joint so that the water in the refueling canal is prevented from entering the annular gap. The prior art shows two main approaches. In one approach, seal rings are bolted into place prior to flooding the canal. In the other approach, a seal ring remains in place at all times. U.S. Pat. No. 4,170,517 to Meuschke et al. shows a permanently installed seal ring extending from a flange on the pressure vessel to a ledge on the primary shield. The seal ring is welded at both ends and curves downwardly from the pressure vessel to the shield to accommodate thermal expansion and contraction of the vessel.
During full power operation, neutrons and gamma rays that penetrate the pressure vessel and are scattered upwardly from the pressure vessel and primary shield wall may make it unsafe for personnel on the operating deck. Thus, when access to the operating deck is required, it may be necessary to bring the reactor down to a 10% operating level.
Even during shutdown, radiation from the decaying fission products and the radio activated shield and piping often necessitates the provision of additional shielding. U.S. Pat. No. 4,090,087 to Weissenfluh discloses the use of removable water bags to provide an annular shield at the level of the pressure vessel joint. While the water bags are presumably effective for their stated purpose, their installation and removal are time consuming and subject personnel to additional exposure.